Magnetron



Od- 1, 1957 c. L. CUCCIA 2,808,568

MAGNETRON Filed March 31, 1954 4 Sheets-Sheet 2 HM M00. SOURCE INVENTOR.

CARMEN L CUCCIA kzwg W4 United States Patent MAGNETRON Carmen L. Cnccia, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application March 31, 1954, Serial No. 419,964

20 Claims. (Cl. 332--) This invention relates to magnetrons, and particularly, to amplitude-modulated magnetrons with means for locking-in the oscillating frequency during variation in the power output. The invention is particularly useful in providing frequency control of ultra-high-frequency magnetrons, such as the multi-cavity and interdigital singlecavity types.

A conventional multi-cavity magnetron comprises an anode structure including an annular array of anode segments or elements connected by cavity resonators, a central cathode coaxially mounted within the anode block, a housing or envelope enclosing the cathode and anode, means for establishing a constant axial magnetic field, means for applying a direct-current voltage between the cathode and the anode, and means for coupling an output load to one of the cavity resonators. In operation, electrons from the cathode are caused by the direct-current electric and magnetic fields to follow curved paths past the gaps between anode elements and induce high frequency voltages between the elements, thus exciting oscillations in the cavity resonators. The cavity resonator circuit is capable of oscillating in a plurality of different modes each having a different resonant frequency. Usually it is desired that the circuit oscillate in the so-called 1r-mode, wherein adjacent anode elements are 11' radians, or 180, out of phase at every instant. Magnetrons usually have alternate anode elements strapped together to favor operation in the 1r-1I1Odt3.

The cavity resonator magnetron is capable of supplying high power at ultrahigh-frequencies with high efficiency. Its geometry and construction are simple and straightforward, making the tube very attractive for application in certain U. H. F. systems. However, the systems which have found a place for the cavity resonator magnetron are principally pulse and continuous-wave systems. It is a diflicult tube to incorporate into an amplitude-modulation system because of its tendency to shift its operating frequency with changes in magnetron current or load impedance. The undesired variation of magnetron frequency as a function of magnetron current is called pushing. The undesired variation in frequency as a result of variation in load impedance is called pulling. The angular frequency of an operating cavity resonator magnetron can be written:

where w =the angular resonant frequency of the unloaded, nonoperating anode circuit w =th6 pushing angular frequency shift, due to the presence of the electron space charge w =the pulling angular frequency shift, due to the coupling of the external load to the anode circuit.

Several systems have been demonstrated for utilizing a cavity resonator magnetron in an amplitude-modulation transmitter using injection-locking apparatus associated with the output line or otherwise coupled to the cavity resonator anode circuit of the magnetron to elim- 2,808,568 Patented Get. 1, 1957 inate frequency variation due to amplitude modulation or variation in load impedance. Several of such systems are disclosed and claimed in the copending application of L. L. Koros, Serial No; 177,455, filed August 3, 1950, now abandoned.

In my copending application Serial No. 412,179, filed February 24, 1954, I disclosed a magnetron having radio frequency control electrodes around the usual central cathode for establishing radio frequency control electric fields in the part of the interaction space near the cathode to lock-in the oscillating frequency at a desired value during variation in power output. Experiments proved that a magnetron could be locked-in with this arrangement of control electrodes. However, a disadvantage inherent in the arrangement is that, since the control electrodes are in the path of the electrons from the central cathode, they cause a deteriorating influence on the performance of the magnetron in that they interfere somewhat with the setting up of the space-charge configuration near the cathode that is vitally necessary for effective control of the oscillating frequency.

The principal object of the present invention is to devisenew and improved means for controlling the oscillating frequency of a magnetron.

Another object is to provide a magnetron with improved means for establishing frequency-controlling radio-frequency electric fields in the region near the electron source.

Still another object of the invention is to provide a magnetron with improved means for minimizing the electronic loading caused by the magnetron electron flow on a lock-signal source coupled thereto.

Yet a further object of the invention is to provide a magnetron with improved means for reducing the amount of injection-lock power necessary to attain a prescribed lock-bandwidth.

Another object of the invention is to permit amplitudemodulation of an oscillating magnetron without attendant frequency modulation.

These and other objects are achieved, in accordance with the present invention, by replacing the usual central cathode of a magnetron by a substantially co-planar array of alternate emissive and non-emissive control electrodes positioned on the surface of a cylinder concentric with the anode of the magnetron and providing means for establishing radio-frequency control voltages between adjacent control electrodes to set up a traveling- Wave control electric field on said array. In two embodiments to be described, by way of examples, the emisslve and non-emissive control electrodes are mounted at one end on the two radio frequency terminals of a control cavity resonator within the tube envelope adjacent one end of the central interaction space. An external transmission line terminal is coupled to the control resonator and sealed through the tube envelope for coupling to an external injection-locking source. In one of these embodiments, means are provided for direct-current separation of the control electrodes to permit the application of different direct-current potentials to the emissive and non-emissive electrodes. 7 In another embodiment, the control electrodes are mounted directly on the two conductors of a transmission line terminal that is similarly sealed through the envelope.

Experiments have shown that by use of the travelingwave grid of the invention the power output of a magnetron can be varied over a very wide range without appreciable frequency modulation.

The invention will be best understood from the following detailed description with reference to the-accompanying drawings, in which: a

Fig. l is a schematic view showing how the electrons in the interaction space of a conventional multi-cavity magnetron are formed into rotating space-charge spokes;

Fig. 2 is a schematic view showing the effective region of influence of the anode electric field and the potential region of influence of a control electric field near the cathode in a coaxial-cathode type magnetron;

Fig. 3 is a schematic view showing the equivalent circuit for an operating magnetron embodying the present invention;

Fig. 4 is an axial sectional view of a vane-type, multicavity magnetron embodying the present invention;

Fig. 5 is an enlarged fragmentary transverse sectional view taken on the line 5-5 of Fi 4; a

Fig. 6 is a fragmentary transverse sectional view of a modified control electrode arrangement;

Fig.7 is an exploded axial sectional view of a modified control resonator mount;

Fig. 8 is a fragmentary axial sectional view of a control resonator structure with insulation between terminals;

Fig. 9 is a graph showing the relation between locking bandwidth and locking power for different values or direct-current bias voltage;

V Fig. 10 is a view similar to Fig. 4 of a direct-drive" magnetron embodying the invention; and

Fig. 11 is a fragmentary transverse sectional taken on theline 1111 of Fig. 10.

Referring now to the drawing in detail, Fig. 1 shows how the electrons from the cathode in a conventional oscillating multi-cavity magnetron are controlled by the induced traveling-wave electric field Ee associated with the anode elements A, and are caused to form spacecharge spokes S of suitable phase rotating around the anode in the interaction space between the cathode and anode. 7

As shown in Fig. 2, the normal region of influence of the anode electric field E91 does not-extend all the way to the cathode surface, but instead, leaves a relativelysmall annular region near the cathode that is substantially free of influence by the anode field. In accordance with the present invention,.an independent traveling-wave control electric field E9 .is set .up in this inner region, for controlling or locking-in the oscillation frequency of the magnetron during variations in operation normally leading to pushing or pulling. The electrons, on their way to the anode, encounter first E9 and then'E with the'possibility of B9 persisting into the E0, field region. From a circuit standpoint, the effect of E9 on the electron cloud willbe to produce electron velocity modulation which will yield a voltage across the anode vane tips-of the same frequency as the injection signal. The

frequency of the injection signalsource may be the same 7 as the desired operating frequency of the magnetron, or otherwise harmonically related thereto.

'Fig. 3 shows schematically the equivalent circuit of an oscillating magnetron with an injection or lock-signal source coupled to the magnetron in such manner as to establish a traveling-wave control electric field in the field region E9, of Fig. 2, and thereby induce lock-signal voltagesacross the magnetron vane tips. The admittance of the magnetron anode circuit is expressed as GM+jBM, that of the magnetron load as GL-l-jBL, and that of the electron cloud as Ge-j-jBe.

Figs. 4 and 5 show an embodiment of the present invention as it was incorporated in 'a demountahle vanetype RCA-development magnetron designed for onekw. output at a frequency of about 900 mc. The tube comprises an anode structure 1, comprising anou'ter cylindrical wall 3 which also serves :as a part of the'vacuum envelope of the tube, withinwhich are radially mounted in an annular array a plurality'nf' radially extending anode vanes 5, which terminate at their inner ends on the surface of a cylindrical electron interaction space 7. The vanes 5 and the portions of the wall 3 between the'vanesprovide twelve cavity resonators 8 coupled between the inner ends of the vanes 5, which inner ends constitutethe anode ele- 4 ments of the tube. The cavity resonators 8 constitute a resonant anode circuit. The ends of the cylindrical wall 3 are closed by end plates 9 and 11 which complete the vacuum envelope. The lower end plate 9 is apertured at 13 to receive a tuning mechanism to be described. Means are provided for connecting the inner ends of alternate vanes 5 together, for 1r-1Tl0d6 operation, in the form of concentric rings 15, 17, 19 and 21. A tuning member 23, mounted below the anode vanes 5, is formed with concentric grooves .25 which .provide concentric ring portions 27 which are inserted between the strapping rings 15, 17, 19 and 21 to change the capacitance therebetween, as shown in Fig. 4. The tuning member 23 is movably mounted in the tube by means of a bellows 29 sealed vacuum tight to the aperture 13 in the bottom plate 11. The tuning member 23 is movable axially with respect to the anode by means of a threaded stud 31 and a nut 33 which bears against a supporting plate 35 mounted on the end plate 11. An apertured plate 37 is mounted within the wall 3 around the tuning member 23 and adjacent to the vanes 5 and serves as an end plate for the cavity resonators 8.

In accordance with the present invention a hollow cylindrical array of control electrodes 39 is axially mounted within the interaction space 7. This array, in the example shown in Figures 4 and 5, consists of six elongated tubular cathodes 41 and six non-emissive rods 43 alternatively positioned in a cylindrical array within and coaxial with the space 7 and spaced from the anode vanes '5. The control electrodes 39 are mounted at one end on the two terminals of a control cavity resonator 45, which in turn is mounted within the cylindrical wall 3 adjacent one end of the vanes 5, by means of an outwardly extending-flange 47 mounted by ceramic insulators 49 and 51, mounting screws 53, and supporting studs-55 on the end plate 11. The resonator 45 comprises a hollow drumshaped metal body 57 to which are connected an inner tubular conductor 59 and an outer tubular conductorol. The cathodes 39 are mounted on the lower end of the inner conductor 59, and the non-emissive rods 43 are mounted on the lower end of the outer conductor 61, as clearly shown in Fig. 4. Hence, the cathodes and the non-emissive rods are conductively connected together for direct currents by the metal walls of the resonator 45 and are tlrus maintained at the same direct-current potential. A conventional heater 63 is provided within each of the cathodes 41 for heating the cathodes to electron emitting temperatures. Heater leads 65 and 67 are brought out from each of the heaters 63 to terminal blocks .69 and 71 mounted on the outside of the resonator 45. One heater .lead for each cathode is connected to thecathode, and the terminal blocks 69 connected thereto are mounted in contact with the resonator. The other terminal block 71 is insulated from the resonator by suitable ceramic insulators. .Leads 73 and 75.from the terminal blocks 69 and 71, respectively, are brought out through .a seal 77 in the upper plate 11 for connection 'to external power sources. The resonator 45 is provided with an aperture 79 through which extends a coupling loop 81 connected to the inner and outer conductors 83 and 85 of a coaxial line terminal 87 sealed through the upper plate 11. The terminal 87 is adapted to be con nected to a standard radio-frequency lock-in source for exciting the resonator 45 at a desired frequency. The resonant frequency of the resonator 45 may be the same as the desired operating frequency of the magnetron, or otherwise harmonically related thereto. Tuning means, such as a screw 88,-may be provided for tuning the resonator45.

As shown in Figure 4, the resonator 45 is provided with a cooling channel 89 surrounding the resonator proper, for cooling the resonator and the control rods 39 connected thereto. Cooling fluid is circulated through the channel 89 by' means of inlet and outletpipes'91, only one of which is shown in the drawing. The pipe 91 is mounted on a tubular assembly 93, which is flexibly connected in insulating relation to the plate 11 by means of a bellows 95 and a dielectric ring 97.

Suitable means, such as a coaxial output terminal 99, is coupled to one of the resonators 8 for extracting output energy therefrom. Means, such as a pair of magnet poles 101, are provided for establishing a coaxial magnetic field extending through the interaction space 7.

In operation, the cathodes 41 and the anode structure 1 are connected, by means of the cathode lead 73 and the external envelope wall, to a direct-current voltage source 103 in series with an amplitude-modulating voltage source 105. In operation, electrons from the cathodes 41 accelerated toward the anode by the source 103 are caused by the axial magnetic field to follow curved paths in the interaction space 7. In the absence of a frequency-locking signal applied to the control resonator 45, the electrons from the cathodes 41 will induce voltages on the anode segments 5 which in turn will cause the space charge to form spokes similar to those shown in Fig. 1, as in conventional magnetrons having a cylindrical central cathode. In accordance with the invention, when the control resonator 45 is excited by a radio-frequency locking source of suitable frequency, radio-frequency control voltages will be established between adjacent control electrodes 39 and produce a radio-frequency control wave traveling around the cylindrical array of electrodes 39. These radio-frequency control fields between each cathode and the adjacent control rods 43 act upon the electrons in such manner that some electrons are caused to speed up in their curved paths and some are retarded. The net result is, therefore, that the time at which the electrons will reach the anode will be affected. Thus, electrons that have been given increased initial velocity will reach the anode sooner than those electrons which have been retarded by the traveling field. Therefore, a type of velocity modulation is instituted in the electron stream so that electrons with different velocities travel through the stream to the vane tips. As a result of field velocity modulation, the electrons appearing at the tips of the space charge spokes will form variations in electron densities which correspond to the velocity modulation produced by the control field near the cathode. This variation in electron density will cause an additional component in electron density in the vicinity of the vane tips which will correspond to the frequency and phase of the traveling wave near the cathode and will introduce into the resonant structure a locking component of circulating current that will lock the frequency of the magnetron according to known principles of injection locking.

Figure 6 shows a modification of the embodiment shown in Figures 4 and 5, in which all of the electrons may start at the same cathode radius. In this embodiment the cathode comprises a cylindrical member 107 coaxially mounted within the interaction space '7 and having longitudinal electron emitting surfaces 108 separated by grooves 109 in each of which is mounted a control rod 111. The remainder of the structure would be the same as that shown in Figs. 4 and 5, the cathode 1117 being mounted on the inner conductor 59 and the rods 111 being mounted on the outer conductor 61, of the resonator 45. Conventional means are provided for heating the cathode 167 to electron emitting temperature. The operation of the embodiment shown in Fig. 6 is similar to that described above for Fig. 4.

Fig. 7 shows a modification of Fig. 4 in which the.

control electrodes 39 and resonator 45 form a separate unit or cartridge 113 adapted to be inserted vw'thin a supporting member 115 having an outwardly extending flange 117, which is mounted on the plate 11 in the same manner as flange 47 of Fig. 4. In this embodiment a cooling channel 119 is formed in the member 115. The remainder of the structure in Fig. 7 is substantially the same as that of Fig. 4. An advantage of this structure is that the grid-cathode structure can be built in cartridge form in several different types of geometries and easily adapted for testing, or for replacement of structures in the case of filament burn-out or emissive coating deterioration.

Experiments conducted on the structure shown in Fig. 4 showed conclusively that frequency locking by use of a co-planar cylindrical cathode-grid structure was entirely feasible and that looking bandwidth up to and even greater than the pushing range of the magnetron can be achieved, in that there was no noticeable deterioration in the high efficiency inherent in the magnetron during the use of the traveling-wave type cathode-grid structure. One aspect was noticed however, this aspect being the tendency for power saturation to take place with regard to input power to the control cavity resonator 45. After a certain value of locking bandwidth had been obtained, despite the continued introduction of power into the control resonator, no additional increase in locking bandwidth could be had. It was felt, therefore, that loading was taking place in the cathode-grid structure due to electrons from the cathodes 41 traveling directly to the non-emissive electrodes 43, creating a conductance which was placing an upper limit on the bandwidth which could be obtained. This aspect was overcome by introducing direct-current separation between the cathodes and the control rods of the cathode-grid structure, as shown in Fig. 8. In that figure, which shows only that part of Fig. 4 which is modified, the body 57 of the resonator 45 is insulated from the inner conductor 121 by a A wave choke 123. This choke comprises a pair of concentric tubular members 125, extending outwardly from the resonator body 57, and a pair of concentric tubular members 127 connected to the upper end of the inner tubular conductor 121 of the resonator 45. The tubular conductor 121 and tubular members 127 are insulatedly supported by means of a flange 129 on the outer member 127 which is mounted between two ceramic rings 131 within a clamping ring 133 attached to the outside wall of the resonator body 57. The total length of the space between the tubular members and 127 is approximately A wavelength so that this space acts as a A wave open section of transmission line, and hence, a radio-frequency short between the inner and outer conductors fo the resonator at the operating frequency. Due to the directcurrent insulation, however, it is possible to apply different direction-current potentials to the cathodes 41 and control rods 43. In this embodiment, because of the presence of the radio-frequency choke on top of the resonator 45, it was found convenient to bring in the coupling loop 135 from the side of the resonator as shown, with a right-angled bend 137 in the input coaxial line terminal 139. In operation, the cathodes 41 and anode structure 1 are connected to a direct-current voltage source 103 in series with an amplitude-modulation voltage source 105, as in Fig. 4. In addition, a direct-current voltage source 141 is connected between the cathode lead 73 and the pipe 91, which is conductively connected tothe control rods 43, for applying a direct-current bias voltage between the cathodes 41 and the non-emissive control rods 43.

In Fig. 9 are shown several curves showing the effect of the use of a direct-current bias voltage between cathodes and the non-emissive control rods on the lock bandwidth as the net locking power is varied. As shown, best results were obtained with the control rods operated at 400 volts negative with respect to the cathodes. It was possible to lock-in the frequency of the tube over a bandwidth of 5 megacycles with about 50 watts of input lock power. At lower bias voltages, the locking bandwidth saturated at 30 to 40 watts of input locking power.

Figs. 10 and 11 show the invention embodied'in a direct drive arrangement in which the radio-frequency locking signal is applied to a coaxial line termnial connected directly to the control electrodes of the magnetron.

An anode structure 143 comprising an outer cylindrical' *7 wall 145, which also serves as part of the vacuum envelope of the tube, within which are radially mounted in an annular array of plurality of radially-extending anode vanes 147 which terminate at their inner ends on the surface of a cylindrical interaction space 149. The vanes 147 and the portions of the wall 145 between the vanes provide cavity resonators 151 between the inner ends of the vanes, which constitute the anode elements of the tube. The ends of the cylindrical wall 145 are closed by end plates 153 and 155 which complete the vacuum envelope. If desired, the anode may be provided with strapping rings and tuning means, as shown in Fig. 4. A cathode-grid structure 157 of generally cylindrical form is coaxially mounted in the interaction space 149. This structure comprises a plurality of radially-extending vane-like non-emissive control elements 159 mounted on a central rod 161 and a plurality of elongated tubular cathodes 163 located between the elements 159 so that the outer portions of the elements 159 and cathodes 163 form a cylindrical array within and coaxial with the interaction space 149. The elements 159 and cathodes 163 are mounted directly on the inner and outer conductors 16S and 167, respectively, of a coaxial-line terminal 169 extending through the upper end plate 155 and insulated therefrom by a dielectric ring 170. The central rod 161 is mounted on the lower end of the inner conductor 165, and the cathodes 163 are mounted on a hollow header structure 171 carried by the lower end of the outer conductor 167 whcih serves as the cathode lead. A tubular extension 172 of the outer conductor 167 serves as outer connector of the coaxial terminal 169. A terminal ring 173 insulatedly mounted within the hollow header 171 is connected to one terminal of each of the heaters (not shown) within the cathodes 163, the other terminal of the heaters being connected to the cathodes 163. The heater terminal ring 173 is provided with a heater lead 175 extending through a seal 177 in the upper end plate 155, for connection to one terminal of a source of heating power. The other terminal of the heater source would be connected to the heater by means of the outer conductor 167. A conventional output terminal 179 is coupled to one of the resonators 151. A pair of magnet poles 181, the upper one of which is centrally apertured to accommodate the coaxial terminal 169, are provided to establish a coaxial magnetic field within the interaction space 149. In operation, the cathode lead 167 and the anode structure 143 are connected to a direct-current voltage source 133 in series with an amplitudemodulating voltage source 185, as in Fig. 4-. A directcurrent bias voltage source 187 is connected between the cathode lead 67 and a tubular Kovar lead-in 18$ for the central rod 165, to provide a direct-current bias between cathodes 163 and the control elements 159. The lead between the source 187 and the lead-in 188 includes a choke 189 in the space between the elements 172 and 183 to avoid radio-frequency short circuiting of the terminal 169.

To aid in the internal cooling of the structure shown in Figure water is introduced into a channel 191 in the outer conductor 167 by way of inlet and outlet water pipes 193. Forced-air cooling of the center conductor 165 may be accomplished by forcing air through an air duct 195. Due to the low heat conductivity of the Kovar lead-in 188, the copper inner conductor 165 is extended through the lead-in 188' and provided with cooling fins 197 opposite the upper end of the air duct 195. A copper tube 199, mounted on the lead-in 188 and providing an inner connector for the coaxial terminal 169, is apertured at 261 to permit the passage of the cooling air to the fins 197. The coaxial connectors 172 and 188 are adapted to be connected to a standard injection-locking source to set up the desired radio-frequency control fields between'the cathodes 163 and control elements 159.

In a copending application of Wellesley J. Dodds, Serial No. 141,006, filed January 27, 1950, now Patent No.

.8 2,784,346, issued March 5, 1957, assigned to the same assignee as the instant application, there is disclosed a magnetron with a cathode-grid structure somewhat similar to that disclosed herein. However, the magnetron in said Dodds application is not an amplitude-modulated oscillator as disclosed and claimed herein, but instead, is a radio-frequency amplifier tube which is non-oscillating except when excited from a source of radio-frequency signal to be amplified. Moreover, said Dodds application does not include a control cavity resonator within the envelope of the magnetron, as in Figs. 4 to 8 herein, nor are the teachings associated with the establishment of the direct-current grid-cathode bias included.

What is claimed is:

l. A magnetron including a hollow anode comprising a plurality of spaced parallel anode elements defining a cylindrical interaction space, a resonant anode circuit coupled to said anode elements, a cylindrical array of elongated emissivc and non-emissive control electrodes positioned within and coaxial with said space, means for applying direct current operating potentials to said anode and control electrodes to generate radio-frequency oscillations on said anode, means coupled to said control electrodes and to said anode for modulating the amplitude of said oscillations, and means coupled to said control elec-' trodes for establishing radio-frequency control voltages between adjacent electrodes, for locking-in the oscillating frequency of said magnetron during variation in power output at a desired operating frequency.

2. A magnetron as in claim 1, wherein said anode circuit comprises a plurality of cavity resonators each connected between two adjacent anode elements.

3. A magnetron as in claim 1, including means for tuning said anode circuit.

4. A magnetron as in claim 1, wherein each of said control electrodes is located substantially opposite the midpoint of the gap between two adjacent anode elements.

5. A magnetron as in claim 1, wherein all of said control electrodes are connected together for direct currents within the magnetron.

6. A magnetron as in claim 1, wherein said emissive control electrodes are insulated for direct-currents from said non-emissive electrodes within said envelope, to permit the application of difierent direct-current potentials thereto.

'7. A magnetron including a hollow anode comprising a plurality of spaced parallel anode elements defining a cylindrical interaction space, a resonant anode circuit coupled to said anode elements, a cylindrical array of elongated emissive and non-emissive control electrodes positioned within and coaxial with said space, means for applying direct current potentials to said anode and control electrodes to generate radio frequency oscillations on said anode, means coupled to said control electrodes and to said anode for modulating the amplitude of said oscillations, and means comprising a control circuit resonant at a control frequency harmonically related to the desired operating frequency of said magnetron coupled to said control electrodes for establishing radio frequency control voltages between adjacent electrodes, for locking-in the oscillating frequency of said magnetron during variation of power output at a desired operating frequency.

8. A magnetron as in claim 7, wherein said control circuit comprises a cavity resonator located within the vacuum envelope of the magnetron.

9. A magnetron as in claim 8, including an external transmission line terminal coupled to said control resonator and adapted to be coupled to an external radio frequency control voltage source.

10. A magnetron as in claim 1, wherein alternate control electrodes are electron-emissive and the other alternate control electrodes are non-emissive.

11. A magnetron including an envelope containing a plurality of spaced parallel anode elements defining a cylindrical interaction space, a resonant anode circuit comprising at least one cavity resonator connected between adjacent anode elements, a cylindrical array of spaced control electrodes coaxially positioned within said space, part of said control electrodes being electron-emissive for supplying electrons within said space, and a control cavity resonator mounted within said envelope and connected between adjacent control electrodes for establishing radiofrequency control voltages therebetween.

12. A magnetron as in claim 11, further including an external transmission line terminal coupled to said control cavity resonator and adapted to be coupled to an external radio-frequency control voltage source.

13. A magnetron as in claim 11, wherein alternate control electrodes are emissive and the other alternate control electrodes are non-emissive.

14. A magnetron as in claim 11, wherein said control electrodes are mounted at one end on said control resonator.

15. A magnetron as in claim 14, including means for circulating cooling fluid over portions of said control resonator, for dissipating heat from said control elec trodes.

16. A magnetron as in claim 14, wherein all of said control electrodes are connected together for direct currents by said control resonator.

17. A magnetron as in claim 11, wherein said control resonator comprises a first conductor on which one set of alternate control electrodes is mounted and a second conductor on which the other set of alternate control electrodes is mounted.

18. A magnetron as in claim 17, including means connecting said conductors for radio-frequency currents but insulating them for direct currents, to permit the application of different direct-current potentials to said two sets of alternate control electrodes.

19. A magnetron including a hollow anode comprising a plurality of spaced parallel anode elements defining a cylindrical interaction space, a resonant anode circuit coupled to said anode elements, a cylindrical array of elongated emissive and non-emissive control electrodes positioned within and coaxial of said space, means comprising a pair of external terminals each connected directly to a difierent set of alternate control electrodes, means connected to said anode and one of said terminals for applying diiferent direct current potentials thereto to generate radio frequency oscillations on said anode and for applying a modulating voltage therebetween to amplitude modulate said oscillations, said terminals being adapted to be coupled to an external radio frequency control voltage source for applying radio frequency control voltages between adjacent control electrodes to lock-in the oscillating frequency of said magnetron during variation in power output at a desired operating frequency.

20. A magnetron as in claim 1, including means for circulating cooling fluid over portions of the supporting structure for said control electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 

